Characterization of Na-X,Na-A,and coal fly ash zeolites and their amorphous precursors by IR,MAS NMR and XPS

By fusion with sodium hydroxide followed by a hydrothermal reaction, fly ash and Alenriched fly ash were converted into Na-X and Na-A zeolites, respectively. The authentic Na-X, Na-A and fly ash zeolites as well as their amorphous precursors have been characterized by IR, ²⁹Si and ²⁷Al MAS NMR, XPS/AES, TG, and comparative ion-exchange studies of Cs and K with Na in zeolite samples. It appears that the same structural unit with a terminal OH, such as a sodalite unit, was formed in the induction period of the hydrothermal reaction for Na-X and Na-A, and then cross-linked through D4R and D6R external linkages to build up the zeolite framework of Na-A and Na-X, respectively.     

粉煤灰制备沸石过程中的微观形态变化及其影响

以酸预处理的粉煤灰为原料,采用水热法合成出类沸石。利用X射线衍射仪（XRD）、扫描电镜（SEM）、X射线荧光光谱仪（XRF）等对所得样品进行微观形态结构和成分分析,研究了粉煤灰合成沸石过程中碱处理对粉煤灰微观形态变化的影响。研究发现水热合成沸石过程中粉煤灰微观形态结构类似于“葡萄干-布丁”形态,其中球状葡萄干-布丁形态结构会使碱不能与埋藏于球内部的石英、莫来石等有效成分充分发生反应,致使大量粒径为2-5μm的粉煤灰球残存,导致沸石产率较低。     

水热法合成NaP1型粉煤灰沸石的性能表征

通过碱性介质中的水热反应,由粉煤灰合成了单一沸石矿物种的NaP1沸石,并对合成产品进行了表征.经粉晶X射线衍射鉴定,合成产物中主要矿物成分为NaP1沸石,另有少量尚未反应的石英和莫来石.在电子显微镜下,粉煤灰颗粒呈球形且表面光滑,而合成产物颗粒表面粗糙.粉煤灰合成沸石含有大量的交换性Ca2 ,且与粉煤灰原料相比,SiO2含量明显减少,Al2O3稍有增加,SiO2/Al2O3比值由3.3降至1.8.红外光谱分析和差热分析证实了合成的粉煤灰沸石中沸石水的存在.NaP1型粉煤灰沸石的阳离子交换容量(CEC)达213 cmol/kg,比表面积达29 m2/g,分别比粉煤灰高约100倍和26倍.     

Effective utilization of waste ash from MSW and coal co-combustion power plant: Zeolite synthesis

The solid by-product from power plant fueled with municipal solid waste and coal was used as a raw material to synthesize zeolite by fusion-hydrothermal process in order to effectively use this type of waste material. The effects of treatment conditions, including NaOH/ash ratio, operating temperature and hydrothermal reaction time, were investigated, and the product was applied to simulated wastewater treatment. The optimal conditions for zeolite X synthesis were: NaOH/ash ratio=1.2:1, fusion temperature=550 degrees C, crystallization time=6-10 h and crystallization temperature=90 degrees C. In the synthesis process, it was found that zeolite X tended to transform into zeolite HS when NaOH/ash ratio was 1.8 or higher, crystallization time was 14-18 h, operating temperature was 130 degrees C or higher. The CEC value, BET surface area and pore volume for the synthesized product at optimal conditions were 250 cmol kg(-1), 249 m(2) g(-1) and 0.46 cm(3) g(-1) respectively, higher than coal fly ash based zeolite. Furthermore, when applied to Zn(2+) contaminated wastewater treatment, the synthesized product presented larger adsorption capacity and bond energy than coal fly ash based zeolite, and the adsorption isotherm data could be well described by Langmuir and Freundlich isotherm models. These results demonstrated that the special type of co-combustion ash from power plant is suitable for synthesizing high quality zeolite, and the products are suitable for heavy metal removal from wastewater.     

污泥-高钙煤系废物制备地聚合物的技术与性能

鉴于地聚合物是一种低碳排放且能源、资源消耗较少的新型胶凝材料,其制备技术倍受关注。采用热活化污泥和高钙煤系废物制备地聚合物,并与采用高钙粉煤灰和矸石制备地聚合物的反应机制与性能进行了对比分析,确定了制备污泥-高钙煤系废物地聚合物的最佳工艺参数。采用XRD、SEM、TG-DTA及FTIR等对制备的地聚合物的原料组成和性能进行了深入分析。研究表明: 经900℃焙烧45 min掺量为40%(＜50 μm)热活化污泥-煤系废物制备的地聚合物具有较好的抗压强度。无定形地聚合物胶凝包裹在球状粉煤灰颗粒周围,有类沸石矿物生成,Al—O/Si—O对称伸缩峰及Si—O—Si/Si—O—Al弯曲振动峰明显。     

Sorption of aqueous phosphorus onto bituminous and lignitous coal ashes

Aiming at the development of a phosphorus removal technology for waste water, phosphate (PO(4)(3-)) retention behavior of bituminous and lignitous coal ashes was investigated using a batch reactor. Ash samples, including fresh and weathered fly and bottom ashes, were studied for their sorption isotherms and reversibility. Fly ashes had a much higher phosphate retention capacity (4000-30,000mgP/kg) than bottom ashes (15-600mgP/kg). Lignitous coal ashes were more capable of retaining phosphate than bituminous coal ashes. The retention process was largely irreversible, and the irreversibility increased with the increase in the retention capacity. Weathering enlarged the retention capacity of the bituminous bottom ash, but substantially lowered that of the fly ash, likely due to the difference in the weather-induced changes between the fly and bottom ashes. Sorption isotherms of fly ashes were found to be adequately represented by the Langmuir model while those of bottom ashes fitted better to the Freundlich model. Concentrations of Ca(2+) and PO(4)(3-) in the aqueous phase were measured at the end of sorption and desorption experiments, and were compared with solubilities of three calcium phosphate minerals. The aqueous solutions were saturated or super-saturated with respect to tricalcium phosphate (Ca(3)(PO(4))(2)) and hydroxyapatite (Ca(5)(PO(4))(3)OH), and slightly under-saturated with respect to amorphous calcium phosphate. It is concluded that precipitation of calcium phosphate is the predominant mechanism for phosphate retention by coal ash under the conditions studied. There is a strong and positive correlation between alkalinity and phosphate sorption capacity. Consequently, acid neutralization capacity (ANC) can be used as an indicator of phosphate sorption capacity of coal ashes.     

Influence of NaOH concentrations on synthesis of pure-form zeolite A from fly ash using two-stage method

Synthesis of pure-form zeolite A were investigated using four concentrations of NaOH solution to dissolve Si source form fly ash, and with the addition of Al source, to prepare initial gel. Experimental results demonstrated, for two-stage method, that NaOH concentrations in initial gel played an important role in synthesis of pure-form zeolite A using fly ash as raw materials. Generally, pure-form zeolite A could be synthesized when following conditions were used: NaOH concentrations, 1.67, 5 and 6.67 M; the synthesis temperature, 100 °C; the corresponding crystallization time, 340, 250 and 190 min. However, a mixture phases of zeolites A and X were obtained at the condition of 3.33 M NaOH solution during various crystallization times. It was found that the higher NaOH concentration was used, the shorter crystallization time of zeolite A was required and the narrower particle size distribution of zeolite A was achieved. In addition, zeolite A submicron-crystals were first synthesized from fly ash using two-stage method in our study.     

Removal of mixed heavy metal ions in wastewater by zeolite 4A and residual products from recycled coal fly ash

The removal performance and the selectivity sequence of mixed metal ions (Co(2+), Cr(3+), Cu(2+), Zn(2+) and Ni(2+)) in aqueous solution were investigated by adsorption process on pure and chamfered-edge zeolite 4A prepared from coal fly ash (CFA), commercial grade zeolite 4A and the residual products recycled from CFA. The pure zeolite 4A (prepared from CFA) was synthesized under a novel temperature step-change method with reduced synthesis time. Batch method was employed to study the influential parameters such as initial metal ions concentration, adsorbent dose, contact time and initial pH of the solution on the adsorption process. The experimental data were well fitted by the pseudo-second-order kinetics model (for Co(2+), Cr(3+), Cu(2+) and Zn(2+) ions) and the pseudo-first-order kinetics model (for Ni(2+) ions). The equilibrium data were well fitted by the Langmuir model and showed the affinity order: Cu(2+) > Cr(3+) > Zn(2+) > Co(2+) > Ni(2+) (CFA prepared and commercial grade zeolite 4A). The adsorption process was found to be pH and concentration dependent. The sorption rate and sorption capacity of metal ions could be significantly improved by increasing pH value. The removal mechanism of metal ions was by adsorption and ion exchange processes. Compared to commercial grade zeolite 4A, the CFA prepared adsorbents could be alternative materials for the treatment of wastewater.     

²²⁶Ra, ²³²Th and ⁴⁰K radionuclides enhancement rate and dose assessment for residues of lignite-fired thermal power plants in Turkey

A total of 77 coal, slag and fly ash samples collected from six thermal power plants were measured by gamma-ray spectrometry. The average (226)Ra activity concentrations in coal, slag and fly ash were measured as 199.8±16.7, 380.3±21.8 and 431.5±29.0 Bq kg(-1), respectively. The average (232)Th activity concentrations in coal, slag and fly ash were measured as 32.0±2.4, 74.0±9.0 and 87.3±9.8 Bq kg(-1), respectively. The average (40)K activity concentrations in coal, slag and fly ash were found to be 152.8±12.1, 401.3±25.0 and 439.0±30.2 Bq kg(-1), respectively. The radium equivalent activities of samples varied from 147.6±8.5 to 1077.4±53.3 Bq kg(-1). The gamma and alpha index of one thermal power plant's fly ash were calculated to be 3.5 and 5 times higher than that of the reference values. The gamma absorbed dose rates were found to be higher than that of the average Earth's crust. The annual effective dose of residues measured in four thermal power plants were calculated higher than that of the permitted dose rate for public, i.e. 1 mSv y(-1).     

Reduction of metal leaching in brown coal fly ash using geopolymers

Current regulations classify fly ash as a prescribed waste and prohibit its disposal in regular landfill. Treatment of the fly ash can reduce the leach rate of metals, and allow it to be disposed in less prescribed landfill. A geopolymer matrix was investigated as a potential stabilisation method for brown coal fly ash. Precipitator fly ash was obtained from electrostatic precipitators and leached fly ash was collected from ash disposal ponds, and leaching tests were conducted on both types of geopolymer stabilised fly ashes. The ratio of fly ash to geopolymer was varied to determine the effects of different compositions on leaching rates. Fourteen metals and heavy metals were targeted during the leaching tests and the results indicate that a geopolymer is effective at reducing the leach rates of many metals from the fly ash, such as calcium, arsenic, selenium, strontium and barium. The major element leachate concentrations obtained from leached fly ash were in general lower than that of precipitator fly ash. Conversely, heavy metal leachate concentrations were lower in precipitator fly ash than leached pond fly ash. The maximum addition of fly ash to this geopolymer was found to be 60wt% for fly ash obtained from the electrostatic precipitators and 70wt% for fly ash obtained from ash disposal ponds. The formation of geopolymer in the presence of fly ash was studied using 29Si MAS-NMR and showed that a geopolymer matrix was formed. X-ray diffraction (XRD) analysis and scanning electron microscopy (SEM) imaging showed the interaction of the fly ash with the geopolymer, which was related to the leachate data and also the maximum percentage fly ash addition.     

Explosibility boundaries for fly ash/pulverized fuel mixtures

Incomplete combustion and subsequent fuel contamination of a waste stream can pose a serious explosion hazard. An example of this type of incident is the contamination of fly ash with unburned pulverized coal. The coal, if present in sufficient quantities in the mixture, can act as a fuel source for a potential explosion. Experiments were conducted in a 20l Siwek explosibility test chamber to determine the minimum fuel contamination of fly ash required to form an explosible mixture. A sample of fly ash from Ontario Power Generation (OPG) (Ont., Canada) was artificially contaminated with Pittsburgh pulverized coal dust (the surrogate used to represent unburned fuel dust). Additionally, the influence of fly ash particle size on the amount of fuel contaminant required to form an explosible mixture was examined. Fine and coarse size fractions of fly ash were obtained by screening the original sample of OPG fly ash.The results show that at least 21% Pittsburgh pulverized coal (or 10% volatile matter) was required to form an explosible mixture of the original fly ash sample and coal dust. The results also illustrate that fly ash particle size is important when examining the explosibility of the mixture. The fine size fraction of fly ash required a minimum of 25% coal dust (12% volatile matter) in the mixture for explosibility, whereas the coarse fly ash required only 10% coal dust (7% volatile matter). Thus, the larger the particle size of the inert fly ash component in the mixture, the greater the hazard.     

Suppression of solidification of calcium-rich incinerator fly ash during thermal treatment for decomposition/detoxification of dioxins

Dioxins and dioxin-like compounds released from municipal and industrial solid waste incinerators have been a serious problem from the viewpoint of environmental pollution control. Since these compounds are concentrated especially on fly ash, supplemental treatment systems to decompose/detoxify them are required after collecting the fly ash either by a bag filter or an electrostatic precipitator. The present work is aimed at developing a heat treatment technique for fly ash, which contains a large amount of calcium (Ca) derived from hydrated lime, at a temperature higher than 500°C by adding chemical additives to prevent the solidification of Ca-rich fly ash. As calcium hydroxychloride (CaClOH) in the Ca-rich fly ash was found to cause solidification of fly ash at high temperatures, sodium hydroxide, mullite and coal fly ash were added as additives prior to the heat treatment. As a result, the additives studied in the present work are effective for decomposing CaClOH and therefore suppressing the solidification of fly ash, and yet they promoted the decomposition/detoxification of dioxins.     

醋酸钙/飞灰复合脱硫脱硝剂热解性能实验研究

为了提高脱硫脱硝效率同时合理利用火电厂产生的飞灰,研究了醋酸钙中加入飞灰制备的复合脱硫脱硝剂的热解性能。对不同条件下制得的醋酸钙/飞灰复合脱硫脱硝剂的热重特性、X射线衍射图谱、比表面积、扫描电镜结果等进行了分析,结果表明:利用醋酸钙和飞灰水合制备的脱硫脱硝剂具有比单纯的醋酸钙和飞灰更大的比表面积;经过900℃煅烧后脱硫脱硝剂孔隙结构最好;脱硫脱硝剂经过900℃煅烧后的热解产物(SiO2、Al2O3、Fe2O3和CaO)有助于脱除烟气中的硫氧化物和氮氧化物。     

Conversion of high silicon fly ash to Na-P1 zeolite: Alkaline fusion followed by hydrothermal crystallization

In this research, we converted high silicon fly ash to a high ion-exchange capacity zeolite using a two stages conversion process. Alkaline fusion was applied to collapse the fly ash crystalline phases and release Si content. Then Si/Al ratio of the synthesis sol adjusted with appropriate amount of industrial grade materials. A synthesis solution with the molar ratio of 2.2 SiO₂:Al₂O₃:5.28 Na₂O:106 H₂O was hydrothermally crystallized to Na-P1 zeolite at 120 °C for 4 h. The as-synthesized zeolite characterized by means of X-ray diffraction, infrared spectroscopy, scanning electron microscopy and thermal analysis. Cation exchange capacity of the zeolite was determined using ammonium acetate method. The zeolitization remarkably improved the cation exchange capacity of the final product (e.g. 3.23 meq/g in comparison to the raw fly ash ～0.005–0.02 meq/g).Due to the high CEC and sufficient whiteness of the final product, we suggest that the as-synthesized zeolitic powder is a potential candidate as a detergent builder.     

Modification of phase evolution in alkali-activated blast furnace slag by the incorporation of fly ash

The microstructural evolution of alkali-activated binders based on blast furnace slag, fly ash and their blends during the first six months of sealed curing is assessed. The nature of the main binding gels in these blends shows distinct characteristics with respect to binder composition. It is evident that the incorporation of fly ash as an additional source of alumina and silica, but not calcium, in activated slag binders affects the mechanism and rate of formation of the main binding gels. The rate of formation of the main binding gel phases depends strongly on fly ash content. Pastes based solely on silicate-activated slag show a structure dominated by a C–A–S–H type gel, while silicate-activated fly ash are dominated by N–A–S–H ‘geopolymer’ gel. Blended slag-fly ash binders can demonstrate the formation of co-existing C–A–S–H and geopolymer gels, which are clearly distinguishable at earlier age when the binder contains no more than 75 wt.% fly ash. The separation in chemistry between different regions of the gel becomes less distinct at longer age. With a slower overall reaction rate, a 1:1 slag:fly ash system shares more microstructural features with a slag-based binder than a fly ash-based binder, indicating the strong influence of calcium on the gel chemistry, particularly with regard to the bound water environments within the gel. However, in systems with similar or lower slag content, a hybrid type gel described as N–(C)–A–S–H is also identified, as part of the Ca released by slag dissolution is incorporated into the N–A–S–H type gel resulting from fly ash activation. Fly ash-based binders exhibit a slower reaction compared to activated-slag pastes, but extended times of curing promote the formation of more cross-linked binding products and a denser microstructure. This mechanism is slower for samples with lower slag content, emphasizing the correct selection of binder proportions in promoting a well-densified, durable solid microstructure.     

A study on the chemical and mineralogical characterization of MSWI fly ash using a sequential extraction procedure

The presence of heavy metals in municipal solid waste incineration (MSWI) fly ash is of environmental concern due to their leaching potential in landfill environments. Sequential chemical extraction was performed on fly ash samples from a large-scale municipal solid waste incineration plant in East China. The transformation of the mineralogical species of fly ash during the sequential extraction was studied using X-ray fluorescence (XRF) and X-ray powder diffraction (XRD). The leaching behavior of heavy metals such as zinc, lead, cadmium and copper in MSWI fly ash was considered to have a dependency relationship with the components of calcium, such as aphthitalite, calcite, anhydrite and calcium aluminate or calcium aluminosilicate.     

Synthesis of calcium phosphate hydrogel from waste incineration fly ash and bone powder

Waste incineration fly ash and bone powder could be successfully recycled into calcium phosphate hydrogel, a type of fast proton conductor. Various properties of the intermediate and calcium phosphate hydrogel from them were characterized and compared with that from calcium carbonate reagent. It was found that the intermediate from the incineration fly ash and calcium phosphate glass was more brittle than that from bone powder and calcium carbonate reagent. The electric conductivity of crystallized hydrogel obtained from all raw materials increases exponentially with temperature. However, the crystallized hydrogel from incineration fly ash has lower electric conductivity and lower crystallinity than that from bone powder and the reagent. Moreover, the difference in electric conductivity between these crystallized hydrogels decreases with temperature. Compared with using the reagent as a raw material, bone powder provides a 25% reduction in the usage of H(3)PO(4) to acquire the crystallized hydrogel which has the highest conductivity. These experimental results suggest that the incineration fly ash and bone powder are useful calcium sources for the synthesis of calcium phosphate hydrogel.     

Characteristics and the behavior in electrostatic precipitators of high-alumina coal fly ash from the Jungar power plant, Inner Mongolia, China

In China, flue gases emitted by coal-fired power plants are mainly cleaned using electrostatic precipitators (ESPs). However, based on observations, there is a decrease in the collection efficiency of ESPs in some power plants after burning Jungar coal in Inner Mongolia. In order to find the mechanism of coal fly ash escaping from ESPs, the size distribution, resistivity, and cohesive force of particulate matter samples from Jungar coal-fired power plants in China were measured using a Bahco centrifuge, a dust electrical resistivity test instrument, and a cohesive force test apparatus invented by the authors. Experiments were carried out to determine the chemical composition and current-voltage curve of fly ash under operating ESPs. The Al₂O₃ content in fly ash was found to reach more than 50%, with the size distribution showing a higher content of PM2.5 and PM10 in high-alumina coal fly ash than in other coal fly ashes. The resistivity of high-alumina coal fly ash was recorded at over 10¹² Ω cm, but this did not result in a clear back corona. The cohesive force of high-alumina coal fly ash was very little. It was sensitive to smoke speed in the electric field, facilitating dust re-entrainment.     

Effective use of fly ash slurry as fill material

A lot of effort has been put into increasing coal ash utilization; however, 50% of total amount is disposed of on land and in the sea. Several attempts have been reported recently concerning slurried coal fly ash use for civil engineering materials, such as for structural fill and backfill. The authors have studied this issue for more than 15 years and reported its potential for (1) underwater fills, (2) light weight backfills, and (3) light weight structural fills, through both laboratory tests and construction works. This paper is an overview of the results obtained for slurry, focusing on the following. (1) Coal fly ash reclaimed by slurry placement shows lower compressibility, higher ground density, and higher strength than by the other methods. This higher strength increases stability against liquefaction during earthquake. (2) Higher stability of the fly ash ground formed by slurry placement is caused by higher density and its self-hardening property. (3) Stability of fly ash reclaimed ground can be increased by increasing density and also by strength enhancement by cement addition. (4) Technical data obtained through a man-made island construction project shows the advantages of fly ash slurry in terms of mechanical properties such as higher stability against sliding failure, sufficient ground strength, and also in terms of cost saving. (5) Concentration in leachates from the placed slurry is lower than the Japanese environmental law. (6) In order to enlarge the fly ash slurry application toward a lightweight fill, mixtures of air foam, cement and fly ash were examined. Test results shows sufficient durability of this material against creep failure. This material was then used as lightweight structural fill around a high-rise building, and showed sufficient quality. From the above data, it can be concluded that coal fly ash slurry can be effectively utilized in civil engineering projects.